While the soma of an animal ages and ultimately dies, the germ line remains effectively immortal by passing heritable information to the next generation. The fitness of the organism absolutely depends on the high-fidelity transmission of this information;therefore, germ cells are uniquely protected against aging-related damage. The protection of the germ cells is conferred at their initial segregation, often through the asymmetric inheritance of a specialized cytoplasm, and maintained long-term by association with a gonadal niche. A comprehensive understanding of the niche-germ cell relationship will illuminate mechanisms that suppress the aging process. The long-term goal of our research is to understand the transcriptional networks involved in germ cell-niche maintenance. The objective of this proposal is to investigate the function of a putative Notch-Delta pathway member, SpMibL, enriched in the presumptive germ cells called small micromeres (SMMs) of the sea urchin Strongylocentrotus purpuratus. These cells are allocated early in early development and are used to construct adult tissues during metamorphosis, including the germ line. Surprisingly, if the SMMs are ablated, the embryo can compensate by reprogramming previously committed lineages. We hypothesize that SpMibL is an upstream regulator of SMM allocation, and also critical for reallocating new SMMs following ablation. SpMibL achieves this by directly repressing delta ligand in the SMMs, establishing a molecular asymmetry;SMMs becoming the notch expressing, receiving cells, and their neighbors become the delta expressing, niche cells. To test this hypothesis, we will knock down and overexpress SpMibL in vivo, analyze the resulting phenotype, and identify downstream transcriptional targets. We will also test whether SpMibL, as well as other known factors are induced in other lineages following SMM ablation. Our results will provide fresh molecular detail on how the germ line niche is maintained, and germ cells can be programmed from previously committed cells.